建筑材料吸水过程中毛细管系数与吸水率关系的理论分析

非饱和建筑材料的毛细吸水过程是造成建筑材料性能劣化的重要因素,它还将引起钢筋锈蚀。本文基于Darcy非饱和流体理论建立了建筑材料的吸水扩散模型,并推导出了非线性扩散方程的近似解析解。根据水分扩散系数与含水量的关系,建立了毛细管系数与吸水率之间的理论关系表达式,据此可通过累计吸水量试验预测水分渗透深度。与文献中试验结果的比较说明,本文提出的理论方法是可行的。

低渗透油藏毛细管压力动态效应

通过实验测定和数值模拟研究,分析低渗透油藏油水两相渗流毛细管压力动态效应,以明确其对水驱油特征的影响规律。建立了动态毛细管压力测量实验装置,对动态毛细管压力和含水饱和度变化率之间的关系进行了测定,并求得低渗透岩心的毛细管动态系数。基于测定结果建立了一维水驱油渗流数学模型,并用其研究了注水过程中动态毛细管压力对低渗透油藏水驱油渗流规律的影响。结果表明,低渗透油藏毛细管压力的动态效应非常显著,不能忽略;动态毛细管压力和湿相饱和度变化率之间近似为线性关系,且不同渗透率岩心的毛细管动态系数大小不尽相同,岩心渗透率越低,毛细管动态系数值越大,毛细管动态效应越明显,启动压力梯度越大。认为毛细管压力动态效应是造成低渗透油藏油水两相流动符合非达西渗流规律的原因之一,会对水驱油前缘的推进速度、沿程压力分布、见水时间、产油量、采出程度等开发指标产生影响。

环境气压降低对水体表面张力系数的影响

表面张力是液体接触气体时的基本特性。设计了低气压下表面张力系数测量装置,研究了气压降低对水体表面张力系数的影响,提出了低气压下表面张力系数计算公式。研究表明,相同水体温度下,气压与水体表面张力系数间呈良好的线性关系;气压越低,表面张力系数越大。气压降低时,气压和温度对水体表面张力的影响是基本独立的。

微型管中液滴流动的三维数值模拟

对于微型设备中的低雷诺数流动,毛细力和黏性力起主导作用．应用相场方法,引入自由能泛函,研究了二相流体在微型管中流动问题及表面浸润现象,并给出了微型管中二相流体的无量纲输运方程．针对方形微管道,利用差分法给出了输运方程的数值求解方法．最后,模拟了方形直管中的液滴流动和变形的过程,并给出了液滴前后压力差与其它主要物理参数之间的变化关系．结果表明,压力差随液滴半径增大而增加,而随毛细管系数的增大而减小．

Experimental investigation of thermal performance characteristics of sintered copper wicked and grooved heat pipes:A comparative study

Heat pipes are most frequently used for thermal management solutions.Selection of right type of heat pipe for a specific scenario is utmost necessary for best outcomes.The purpose of this research is comparison of thermal performance characteristics of sintered copper wicked and grooved heat pipes,which are mostly used types of heat pipes.Distilled water filled heat pipes were tested through experimentation in gravity assisted position.Experimental outcomes have been compiled in terms of capillary pressure,operating temperature,thermal resistance and heat transfer coefficient.Capillary pressure is high in sintered heat pipes compared to grooved heat pipes irrespective of groove dimensions.Grooved heat pipes have lower operating temperature compared to sintered heat pipes at the same heat load.At 8 W,compared to sintered heat pipes,grooved heat pipes have 8.24% lower condenser surface temperature,4.41% lower evaporator surface temperature and 7.79% lower saturation temperature.Thermal resistance of sintered heat pipe is much lower than grooved heat pipe.The maximum relative difference of 63.8% was observed at 8 W.Heat transfer coefficient of sintered heat pipe was observed double compared to grooved heat pipe at 8 W heat load.Thermal resistance and hence heat transfer coefficient of sintered heat pipe change almost in a linear manner with respect to heat load but unexpectedly turning point is observed in thermal resistance and heat transfer coefficient of grooved heat pipe.Grooved heat pipes attain equilibrium much earlier compared to sintered ones.Varying heat loads from 4 to 20 W causes variation in equilibrium establishment time from 7 to 4 min for grooved and from 10 to 7 min for sintered heat pipes.

The Moisturein Capillaries of Building Materials

The behavior of building materials in constructions of civil structures is influenced by the surrounding moisture and it is a crucial for intensively examined field of the construction physics. Most standard building materials are characterized by a porous structure, which results in the ability to receive water in a liquid as well as gaseous form in the inner pores. The water fills the storage space of pores under certain conditions; it is transported and transferred back to the surroundings. Many technical studies show that the moisture monitoring is prevailingly based on experiments. Previous calculating methods introduced, e.g., by Glaser, which became the basis for the standard calculations in many European countries in the 1960s, are not always sufficient with respect to the demands of the civil structures. The moisture influences thermo-insulating properties of the material. By a change of the thermo insulation properties of the construction also the thermal and diffusion scheme of the construction is changed and its thermal resistance is decreasing. Faults in the thermo-technical projects occur when thermal conductivity coefficient L values for material in a dry state are substituted.The aim of the research is to determine the capillary conductivity coefficient as a characteristic material moisture parameter of the building materials by the means of a non-destructive method using the experimentally assembled apparatus developed at the Department of Civil Engineering, Brno University of Technology. Keywords： Capillary conductivity coefficient, moisture transfer, EMWR （electromagnetic microwave radiation）, diffusion

Asymmetric breakup of a droplet in an axisymmetric extensional flow

The asymmetric breakups of a droplet in an axisymmetric cross-like microfluidic device are investigated by using a three-dimensional volume of fluid(VOF) multiphase numerical model. Two kinds of asymmetries(droplet location deviation from the symmetric geometry center and different flow rates at two symmetric outlets) generate asymmetric flow fields near the droplet, which results in the asymmetric breakup of the latter. Four typical breakup regimes(no breakup, one-side breakup, retraction breakup and direct breakup) have been observed.Two regime maps are plotted to describe the transition from one regime to another for the two types of different asymmetries, respectively. A power law model, which is based on the three critical factors(the capillary number,the asymmetry of flow fields and the initial volume ratio), is employed to predict the volume ratio of the two unequal daughter droplets generated in the direct breakup. The influences of capillary numbers and the asymmetries have been studied systematically in this paper. The larger the asymmetry is, the bigger the oneside breakup zone is. The larger the capillary number is, the more possible the breakup is in the direct breakup zone. When the radius of the initial droplet is 20 μm, the critical capillary numbers are 0.122, 0.128, 0.145,0.165, 0.192 and 0.226 for flow asymmetry factor AS= 0.05, 0.1, 0.2, 0.3, 0.4 and 0.5, respectively, in the flow system whose asymmetry is generated by location deviations. In the flow system whose asymmetry is generated by two different flow rates at two outlets, the critical capillary numbers are 0.121, 0.133, 0.145, 0.156 and 0.167 for AS= 1/21, 3/23, 1/5, 7/27 and 9/29, respectively.

玻化微珠保温材料吸水性研究

非饱和状态下的玻化微珠保温材料具有很高的吸水能力,严重影响了其保温性和耐久性。制备以玻化微珠为骨料的保温材料,通过吸水性试验,结合吸水率S和毛细管系数k之间的关系式,即k=CS/ω,确定了二者的数值(t<60min)。结果表明:吸水率S和毛细管系数k随材料孔隙率的增大而增大,吸水性与气孔的连通性和骨料表面的开口孔有关,材料内部的连通孔、骨料表面的开口孔的孔径大、数量多,对应的吸水率S和毛细管系数k值大,材料的吸水性强。